Hormonal Regulation of Male Reproductive Function Jonathan Manalang Bio 260: Tuesday Lab Carlene Tonini-Boutacoff May 17th, 2016

The Hypothalamic-Pituitary-Gonadal (HPG) Axis What is it? The hypothalamus, anterior pituitary gland, and the testes are involved in a sequence of hormonal regulatory events Established during puberty Regulates the testosterone and sperm production Gonadotropin-releasing hormone (GnRH), follicle- stimulating hormone (FSH), luteinizing hormone (LH), testosterone, and inhibin are hormones that are involved with HPG axis

Hormone Source Action Gonadotropin-releasing Hormone (GnRH) Hypothalamus Stimulates the anterior pituitary to release gonadotropins (FSH and LH) Gonadotropin: FSH Anterior Pituitary Stimulates sperm production Gonadotropin: LH Stimulates testosterone production Testosterone Testes and sometimes in zona reticularis of adrenal cortex -Onset of puberty that stimulates spermatogenesis, maturation of male reproductive organs and appearance of secondary sex characteristics -Regulates male reproductive organs -Stimulates sperm production Inhibin Testes Serves as “barometer” of the normalcy of spermatogenesis

The Sequence of HPG Axis Events 1. The hypothalamus releases gonadotropin- releasing hormone (GnRH).

The Sequence of HPG Axis Events 2. GnRH binds to pituitary cells (gonadotropic cells), which it stimulates the anterior pituitary to secrete FSH and LH

The Sequence of HPG Axis Events 3. FSH stimulates the sustenocytes to release androgen-binding protein (ABP). ABP can keep the concentration of testosterone in the spermatogenic cells high, and thus, it causes the spermatogenic cells receptive to testosterone.

The Sequence of HPG Axis Events 4. LH binds to the interstitial endocrine cells surrounding the seminiferous tubules, which it causes to release testosterone. In this area, the rising testosterone levels serve as the final trigger for spermatogenesis.

The Sequence of HPG Axis Events 5. Testosterone entering the bloodstream puts a lot of effects at other body sites, and it stimulates maturation of sex organs, development and maintenance of secondary sex characteristics and sex drive (libido).

The Sequence of HPG Axis Events 6. There is feedback inhibition on the hypothalamus and anterior pituitary when there are rising levels of testosterone. Thus, it causes to inhibit the release of GnRH and the gonadotropins (FSH and LH).

The Sequence of HPG Axis Events 7. There is feedback inhibition on the hypothalamus and anterior pituitary when sperm count is high, inhibin is released. This will end the release of FSH and GnRH. However, the inhibin secretion will decline when the sperm counts falls below 20 million/mL.

HPG Axis Balance: The 3 Interacting Sets of Hormones The amount of testosterone and sperm production is based on a balance of 3 interacting sets of hormones. GnRH indirectly stimulates the testes via its effect on FSH and LH release. Gonadotropins (FSH and LH) directly stimulate the testes. Gonadal hormones (testosterone and inhibin) exert negative feedback controls on the hypothalamus and anterior pituitary.

Thank you!! Good luck on your finals, and best wishes in your future!

References Anatomy & Physiology: Openstax College. (2013). Marieb, E. & Hoehn, K. (2013). Human Anatomy & Physiology, ninth edition. Boston: Pearson. What Does Testosterone do? (2014, January). Retrieved May 13, 2016, from http://www.hormone.org/hormones-and-health/what-do- hormones-do/what-does-testosterone-do